This application claims priority to an application entitled xe2x80x9cDevice and Method for Compensating for Distortion in Multi-code Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Dec. 27, 1998 and assigned Serial No. 98-59044, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a multi-code mobile communication system, and in particular, to a device and method for compensating for a distortion occurring during multi-code transmission in a CDMA (Code Division Multiple Access) mobile communication system.
2. Description of the Related Art
The most challenging issue to the rapidly developed mobile communication industry is efficient use of limited radio frequency bands. High rate transmission using a multi-code is one of methods for efficiently accommodating future radio multi-services without increasing the spreading band width of the limited radio frequency. The multi-code transmission is a scheme of converting high rate data to several parallel low rate data, assigning an orthogonal code to each parallel code channel on which to transmit the converted parallel low rate data, and then combining the low rate data, prior to transmission.
FIG. 1 illustrates the structure of a typical transmitter in a multi-code mobile communication system. Referring to FIG. 1, four different orthogonal codes are used for data transmission. Since each subscriber uses the same transmitter in structure, a transmitter 111 for transmitting a signal to a specific subscriber will be described by way of example.
In FIG. 1, the transmitter 111 is comprised of a serial-to-parallel converter (SPC) 121, five multipliers 131 to 134 and 151, an adder 141, and an amplifier 161. The SPC 121 converts input serial user information bits b1(t) to parallel four information bits b11 to b14. The multiplier 131 orthogonally spreads the information bit b11 by multiplying the information bit b11 by a first orthogonal code a11. The multiplier 132 orthogonally spreads the information bit b12 by multiplying the information bit b12 by a second orthogonal code a12. The multiplier 133 orthogonally spreads the information bit b13 by multiplying the information bit b13 by a third orthogonal code a13. The multiplier 134 orthogonally spreads the information bit b14 by multiplying the information bit b14 by a fourth orthogonal code a14. The adder 141 sums the spread information bits received from the multipliers 131 to 134 bit by bit. The multiplier 151 PN-spreads the output of the adder 141 by multiplying the output of the adder 141 by a first PN code PN1. The amplifier 161 amplifies the PN-spread signal received from the multiplier 151.
As described above, the typical transmitter converts a signal provided by a specific user to parallel signals, spreads each parallel signal with a different orthogonal code, sums the orthogonally spread signals, spreads the sum with a predetermined PN code, and amplifies the PN-spread signal, prior to transmission. Spreading the parallel signals with different orthogonal codes can be referred to as use of a multi-code.
Meanwhile, signals output from transmitters 111 to 11N in the above procedure are added in the air and then transmitted to a base station. From FIG. 1, it is noted that AWGN (Additive White Gaussian Noise) is added to the transmission signals of the transmitters 111 to 11N during transmission.
FIG. 2 illustrates the structure of typical receivers 211 to 21N corresponding to the transmitters 111 to 11N using the multi-code scheme. While the plurality of receivers 211 to 21N are shown in FIG. 2, the following description will be conducted on the receiver 211 of a specific user because they are the same in structure.
Referring to FIG. 2, the receiver 211 includes five multipliers 221 and 231 to 234, accumulators 251 to 254, deciders 261 to 264, and a parallel-to-serial converter (PSC) 271. The multiplier 221 PN-despreads an input signal r(t) by multiplying the input signal r(t) by its unique PN code PN1(t). The multipliers 231 to 234 multiply the PN-despread signal by a conjugate signal xcex21ejxcfx86 for channel compensation. Multipliers 241 to 244 multiply channel-compensated signals received from the corresponding multipliers 231 to 234 by corresponding orthogonal codes a11(t) to a14(t), for orthogonal demodulation. The mutually different orthogonal codes are the same as used in the transmitter 111. The accumulators 251 to 254 accumulate the despread signals received from the multipliers 241 to 244 in symbol units. The deciders 261 to 264 subject information bits of the outputs of the corresponding accumulators 251 to 254 to a decision. The PSC 271 converts serial information bits based on the decision results, received from the deciders 261 to 264, to a serial information bit sequence b1(t). The PSC 271 corresponds to the SPC 121 of the transmitter 111.
As shown in FIG. 2, each of the receivers 211 to 21N PN-despreads an input signal and divides the PN-despread signal into four signals as many as the codes used. Then, it multiplies each of the four signals by an orthogonal code, for orthogonal demodulation, accumulates the orthogonally demodulated signal, and subjects information bits of the accumulated signal to a decision. The decided signal is converted to a serial signal, which is information bits obtained in the receiver.
A signal passes th rough a transmitter amplifier with an increased average peak to power ratio in the multi-code transmission scheme, as compared to a single code transmission scheme. In general, a high power amplifier (HPA) corresponding to the transmitter amplifier exhibits non-linear characteristics. Therefore, if the saturation point of the HPA is set to a high level, the non-linear characteristics produces a distortion signal. To reduce the distortion, a transmitter drops the saturation point of its amplifier. The resulting decrease of efficiency of the amplifier, however, causes another problem. Furthermore, considering that power consumption is a dominant factor which determines the performance of a terminal, the above transmitter amplifier cannot be used for the terminal because it increases the power consumption.
A signal is distorted while passing through a transmitter amplifier with a high saturation point and a receiver experiences a low bit error rate (BER) performance. Therefore, the distorted signal should be compensated. In addition, since the distorted signal is likely to have errors during transmission in the air, the receiver is to be provided with an error correcting device.
It is, therefore, an object of the present invention to provide a device and method for efficiently compensating for the HPA-caused distortion of a received signal in a multi-code mobile communication system.
It is another object of the present invention to provide a device and method for outputting a received signal whose distortion is compensated for using a Euclidean distance in a multi-code mobile communication system.
It is a further object of the present invention to provide a device and method for removing another user signal acting as an interference signal in a multi-code mobile communication system.
These and other objects are achieved by providing a device for compensating for the distortion of an input signal for a receiving device in a multi-code mobile communication system. In the distortion compensating device, each of at least two receivers has a pattern generator for generating pattern signals from symbol data, and a regenerator for spreading the symbol data and pattern signals. Here, each symbol is decided by a multi-code from the input signal. An interference canceler removes a signal output from the other of the two receivers from the input signal of a specific receiver. A distortion compensator receives the symbol data and the pattern signals from the pattern generator, calculates Euclidean distances between the output of the regenerator and the output of the interference canceler, and selects one of the symbol data and pattern signals as input information based on the calculated Euclidean distances.
According to another aspect of the present invention, there is provided a method of compensating for an input signal in a receiving device having receivers for regenerating input signals received from at least two different terminals in a multi-code mobile communication system. In the distortion compensating method, pattern signals are regenerated from symbol data and the symbol data and pattern signals are spread. Here, each symbol is decided by a multi-code from the input signal. Then, a signal output from the other of the two receivers is removed from the input signal of a specific receiver, Euclidean distances are calculated between the output of the receiver and the interference-free signal, and one of the symbol data and pattern signals is selected as input information based on the calculated Euclidean distances.